This invention relates to the control of the timing of transmission time slots within a time frame for a communication system which allocates different time slots within a frame to different communication channels.
Various multiplexing techniques for communications systems are known which involve the division of time frames into time slots for different channels. Time division duplex (TDD) is a technique for establishing full duplex communications having forward and reverse links occurring on the same frequency, but separated in time to avoid collision. Time division multiple access is a scheme whereby transmissions to or from a plurality of users are separated in time in order to avoid conflicts. It is also possible to combine time division techniques with frequency division techniques.
This invention is particularly concerned with interference which may affect a signal transmission during a given time slot, which interference arises from a different communication channel but at a similar frequency and time. Individual communication systems adopt measures for avoiding interference between different communication channels within the system. For example, in the global system for mobile communications (xe2x80x9cGSMxe2x80x9d) each transmission time slot is surrounded by guard times to account for uncertain signal propagation delays between the base station and the user stations, in order to avoid collision of signals from different user stations. By comparing the time of the signal received from the user station to the expected received time, the base station in a GSM system may command the user station to advance or retard its transmission timing. This feature is known as adaptive frame alignment. WO 96/08885 also discloses a frame timing method for a time division system, in which transmission timing is adapted to take account of propagation delays, so as to avoid overlap between signals from multiple users at different distances.
The present invention is concerned more particularly with interference between different communication systems. For example, local area networks may have unlicensed spectrum allocation so that the same frequency band is shared by different local area network communication systems. When these systems are deployed in close proximity to each other (for example DECT communications systems in adjacent office buildings) there may be interference and co-existence problems. In particular, there may be no synchronisation of timing between the two systems so that mobile stations of the two systems may be generating signals at the same frequency and at the same time which may cause interference.
The interference levels may also change over time, as a result of so-called sliding interferers, whereby differences in clock frequencies result in timing relationships between two systems slowly changing. In the DECT system, the presence of sliding interferers is detected by testing for localised corruption of synchronisation data or by using cyclic redundancy check fields. The DECT system then enables handover to a different frequency before channel data is corrupted.
According to the invention, there is provided a method of controlling the timing of a transmission time slot within a time frame, a channel being allocated to the time slot for transmission from a transmitting station to a receiving station, comprising:
transmitting information over the channel in the time slot at an allocated channel frequency;
at the receiving station, measuring an interference level at the channel frequency around the start and around the end of the time slot;
analysing the measured interference levels to detect the presence or absence of interferers at the channel frequency around the start and around the end of the time slot;
if required, adjusting the timing of subsequent time slots for the channel to reduce interference from the detected interferers.
In the method of the invention, transmission slot timing enables dynamic avoidance of interference, which can therefore avoid both intermittent and continuous interferers. This method also enables the throughput of the communication system to be largely unaffected.
The timing of time slots within the time frame other than the time slot for the channel may also be adjusted based on the interference level analysis. At the limit, it is possible to alter the timing of all time slots within a time frame based on one detected interferer, so that the system effectively becomes synchronised with the detected interferer. Thus, the timing adjustment of the time slots may be effected by altering the clock frequency of the system.
Alternatively, the timing adjustment may comprise a shift forward in time or a shift backward in time of an individual time slot within the time frame by a predetermined amount.
The measurement around the start of the time slot may be in the preceding time slot, and the measurement around the end of the time slot may be in the succeeding time slot or in part of those time slots. This enables detection of interference immediately before and after the time slot allocated to the channel of interest. Continuous analysis of the interference levels enables determination of when an interferer is approaching the time slot of interest, for example as a result of unsynchronised system clocks in nearby communication systems.
The measurement may comprise measurement of received signal power substantially at the channel frequency, so that the total effect of other signals to the frequency band of the channel can be detected. Alternatively, the measurement may comprise bit error or signal quality measurement at the start and the end of the time slot, to measure the corruption of bits.
Timing adjustment information is preferably transmitted from the receiving station to the transmitting station, to enable the timing of subsequent transmissions to be altered.
The invention also provides a telecommunication station (a base station or a mobile user station) comprising receiving circuitry for receiving a signal at a channel frequency and in an allocated time slot within a time frame;
measuring means for measuring an interference level, at the channel frequency, of a received signal at different points in time;
analysis means for analysing measured interference levels to detect the presence or absence of interferers at the channel frequency around the start and around the end of the time slot, and for calculating a timing adjustment for the timing of subsequent time slots for the signal in order to reduce interference from detected interferers; and
transmitting circuitry for transmitting time adjustment information.
The measuring means may comprise signal power measurement circuitry.
The invention further provides a telecommunication system comprising a plurality of telecommunication stations, each telecommunication station comprising:
receiving circuitry for receiving a signal at a channel frequency and in an allocated time slot within a time frame;
measuring means for measuring an interference level, at the channel frequency, of a received signal at different points in time;
analysis means for analysing measured interference levels to detect the presence or absence of interferers at the channel frequency around the start and around the end of the time slot, and for calculating a timing adjustment for the timing of subsequent time slots for the signal in order to reduce interference from detected interferers; and
transmitting circuitry for transmitting time adjustment information.